Liquid crystal display devices are widely used as displays of portable information terminals because of their thinness and lightness and also because of their small electric power consumption. The liquid crystal display devices require lighting devices because the liquid crystal panels, which are constituent members of the liquid crystal display devices, are not spontaneous luminous elements. The lighting devices are classified in the so-called back light type, in which light is provided on the back surface of a liquid crystal panel, i.e., on the opposite side of a viewer and the so-called front light type, in which light is provided on the front surface of a liquid crystal panel, i.e., on the side of a viewer. The back light is used in the transmittal liquid crystal panels, and the front light is used in the reflective liquid crystal panels. The reflective liquid crystal panels basically require no lighting device, but recently many of the reflective liquid crystal panels have front panels on the front side because the reflective liquid crystal panels are difficult to view in circumstances of little light.
In the liquid crystal display devices used in portable information terminals, the lighting devices must be thin as well, and to this end, the so-called side light (also called the edge light) lighting device, in which a linear light source, such as a cold cathode tube or others, is disposed together with a reflector (a reflecting plate) on the side of a flat light guide plate of a transparent material, is used. In the lighting device of such side light-type, light emitted from a linear light source is incident on the side surface of the light guide plate directly or indirectly after reflected on the reflector. The light which has entered the light guide plate exits from the upper side of the flat light guide plate in the back light while propagating in the light guide plate to enter the liquid crystal panel disposed on the side of the upper surface of the light guide plate, which is the lighting means. In the front light, the light exits from the lower surface of the flat light guide plate to enter the liquid crystal panel disposed on the side of the lower surface of the light guide plate, which is the lighting means.
The light emitted from a light source, such as a cold cathode tube or others, is usually random polarized light, i.e., non-polarized light, and the light exiting the light guide plate is also random polarized light. The liquid crystal panel usually has a polarizer on the incidence side alone, or on the incidence side and the exit side so that prescribed linear polarized light alone enters the liquid crystal panel. That portion of the random polarized light exiting the light guide plate and entering the liquid crystal panel, which actually enters the liquid crystal panel and is used for display is substantially a half of the random polarized light. The utilization efficiency of the light is low. Higher luminance is required for the displays of the portable information terminals.
Higher luminance of this prior art structure can be provided by increasing the light emission amount of the light source, such as a cold cathode tube or others. However, this unsuitably increases the electric power consumption, especially for the displays of portable information terminals, etc. driven by batteries.
As means for solving this problem, light exiting the light guide plate and entering the liquid crystal panel is formed into linearly polarized light, whereby substantially all the light exiting the lighting device is incident on the liquid crystal panel to be used.
For example, the specification of Japanese Patent Application Unexamined Publication No. Hei 09-73083 (1997) discloses the side light-type lighting device in which a sub-light guide plate, polarization separation means of a cholesteric liquid crystal sheet and phase transforming means of polyvinyl alcohol (PVA) are arranged between a light source and a light guide plate in the stated order from the light source. In this prior art, the polarization separation means of a cholesteric liquid crystal sheet transmits first circularly polarized light and reflects second circularly polarized light, and the phase transforming means transforms the first circularly polarized light into linearly polarized light to make the linearly polarized light incident on the light guide plate. To keep the polarization separation means from being affected by the incident angle dependency, the sub-light guide plate is provided to make light enter the polarization separation means at the same angle. On the other hand, the reflected second circularly polarized light returns to the side of the light source, is reflected on the surface of the light source, a reflector or others to be circularly polarized light having the polarity reversed, i.e., the first circularly polarized light and is transmitted by the polarization separation means.
Accordingly, in the prior art lighting device, more than half of the light from the light source can be introduced into the light guide plate to be used.
However, the above-described prior art lighting device requires the assistant member, such as a sub-light guide plate which controls the incident angle of light on the polarization separation means to keep the polarization separation means, for separating the circularly polarized light from being affected by the incident angle dependency. The above-described reference discloses the structure which does not include such assistant member (the sub-light guide plate), but practically, without the assistant member, the desired effect of improving the utilization efficiency of the light source light cannot be obtained.
The above-described prior art has disadvantages that the lighting device has a large number of members, and accordingly cannot realize smaller sizes and reduced weights and cannot be fabricated at low costs.
In view of the above-described problem, the present invention aims at providing a lighting device having high efficiency of utilizing light source light and including a smaller number of members, and a liquid crystal display device having improved brightness (luminance).